1. Field of the Invention
The present invention relates to an antenna and, more particularly, to a dual band antenna in which a separate matching circuit is not required between a signal source and the antenna, thereby having simple construction, convenient usage, low price and enhanced performance.
2. Description of the Related Art
In a mobile communication system, an antenna serves to conserve transmitting power and to use frequency efficiently. With the rapid development and widespread usage of mobile communications, there are frequent occurrences of capacity saturation in a conventional system. Thus, there is a need for a new system which works well in such environment and an interlock between a conventional system and the new system. For example, interlocks are used between: (i) a Code Division Multiple Access (CDMA) system and a Personal Communication System (PCS) in Korea; (ii) an Advanced Mobile Phone Service (AMPS) system and a PCS in the United States; (iii) a Groupe Special Mobile (GSM) system and a Digital European Cordless Telephone (DECT) system; or (IV) a GSM system and a Digital Communication System (DCS) 1800 system, applying the GSM to band 1,800 MHz in Europe. Such interlock systems are commonly called dual band systems. That is, a dual band system interlocks two different systems having frequency bands different from each other.
In conventional dual-band systems having different antennas for the respective two bands, there exists duplication in material costs which makes miniaturization and weight reduction difficult. Therefore, a dual band antenna usable at two bands has been developed.
U.S. Pat. No. 4,509,056 discloses a multi-frequency antenna employing tuned sleeve chokes. FIG. 1 is a section view illustrating the construction of a monopole antenna operating at dual frequency according to an embodiment of the multi-frequency antenna employing tuned sleeve chokes. As shown in FIG. 1, an outer conductor 6 of a coaxial feed line 2 is connected to a ground plane 20 and an extension 10 of an inner conductor 8 is extended from the ground plane 20 passing through a choke 12i to a radiating section indicated as dimension N. The choke is loaded with a solid dielectric insert 16i and the inner surface of the shell of the choke and the outer surface of the conductor extending through the choke form a quarter wavelength (xcex/4) transmission line. At high frequency, the choke forms a xcex/4 transmission line which prevents coupling between an open end of shell 14i of the choke 12i and the extension 10. At low frequency, the choke 12i is operated not as an isolation element but as a monopole antenna indicated as the entire length P at the low resonant frequency.
The dual band antenna operating as a quarter wavelength monopole antenna at high/low band frequencies has an input impedance Zin as defined in equation 1 and requires a 50xcexa9 matching circuit in the case where it is connected to another circuit of the system. Here, the other circuit means a filter or a radio frequency (RF) amplifier, and when it is connected to the dual band antenna, the performance of the antenna is reduced due to impedance mismatching. Therefore, the 50xcexa9 matching circuit should be connected for preventing the mismatching as described above.
Zin=36+j20xe2x80x83xe2x80x83(1)
Since the above dual band antenna requires a separate matching circuit between a signal source and the antenna, it results in complicated construction, inconvenience of usage, and high price.
It is therefore an object of the present invention to provide a dual band antenna in which a separate matching circuit is not required between a signal source and the antenna, thereby having simple construction, convenient usage, low price and enhanced performance.
To achieve the above objects, an embodiment of the present invention is provided, that is, a dual band antenna for a mobile communication system which includes: a metal tube having an open end; a coaxial feed line having inner and outer conductors, with one portion of the coaxial line inserted into the metal tube. A ground plane is connected to a portion of the metal tube opposite the open end and to the outer conductor of the coaxial feed line. A signal line is electrically coupled to the inner conductor and protrudes from the metal tube at the open end thereof.
Preferably, the dimensions of the metal tube, the signal line, and the coaxial line are selected such that the impedance of the antenna is substantially matched to the impedance of the coaxial feed line over the dual band of operation. Optionally, the metal tube can be filled with dielectric to shorten the antenna length.
In a more specific embodiment, a dual band antenna for a mobile communications system includes a metal tube, a coaxial feed line having one portion inserted into the metal tube, a ground plane connected to a first end of the metal tube and to the outer conductor of the coaxial feed line, and a signal line. The signal line is connected to the inner conductor of the coaxial line at a connection point within the metal tube. The outer diameter of the coaxial line is open at the connection point, thus creating a first radio frequency (RF) choke. The metal tube has a second end that is open to create a second RF choke. The signal line passes through the metal tube and protrudes past the metal tube by a predetermined length. Predetermined values are established for: the length of the coaxial line from the ground plane to the connection point; the length of the signal line from connection point to the open end of the metal tube; the length of the protruded signal line from the open end of the metal tube; the outer diameter of the outer conductor of the coaxial feed line; and diameters of the metal tube and the signal line. These values are selected such that, in an operating frequency band of said antenna, impedance of the antenna substantially matches impedance of the coaxial line, thereby providing a low standing wave ratio on the coaxial line, and obviating the need for a separate matching network.